Method for making permeable film

ABSTRACT

In a method for making a permeable film, a physical force is applied to a film of a first material containing particles of a second material which is different from the first material. The second material has a higher susceptibility to the physical force than does the first material. Applying the physical force to the film affects the particles which in turn affect the first material of the film, thereby increasing permeability of the film.

This application is a continuation of U.S. patent application Ser. No.09/200,385 filed Nov. 24, 1998 U.S. Pat No. 6,188,043.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to manufacture of flexible materials forpackaging and the like. More particularly, it relates to a method formanufacturing permeable films.

2. Background Information

In the packaging industry, allowing oxygen to permeate a film or packageso as to contact a product contained therein often is desired. Forexample, a package utilizing a permeable film can permit oxygen topermeate to a fresh red meat product in the package, thereby allowingthe meat product to oxygenate (sometimes called blooming). This canenhance consumer appeal, and retail vendors of such meat products havebegun to demand this type of capability. Additionally, many types ofproduce require the presence of oxygen to suppress anaerobic spoilage.

To obtain sufficient permeability, films frequently are treated withmechanical perforating mechanisms. Unfortunately, mechanical perforationis expensive and cannot be accomplished easily after film manufacture orpackaging by, for example, retail vendors. Furthermore, providingperforations which are sufficiently small and uniform in size is a notinsubstantial challenge using presently available mechanical perforatingtechniques.

The need remains for an efficient, cost-effective, versatile method ofmaking permeable films.

SUMMARY OF THE INVENTION

According to the present invention, a method for making permeable filmis provided. The film advantageously can be made permeable before orafter packaging, and permeability is provided through small andsubstantially uniform perforations as desired.

Briefly, the present invention provides a relatively uncomplicated,inexpensive method for making a permeable film. The method involvesapplying a physical force to a film of a first material that containsparticles of a second (different) material which has a highersusceptibility to the physical force than the first material.Application of the physical force affects the second material which actson the first material so as to form holes, thereby increasingpermeability of the film. The second material can be particles or flakesderived from one or more of a metal, carbon black, ferromagneticmaterial, or the like. The physical force may be one or more ofinductive heating, infrared heating, magnetic force, ultrasonicexcitation, microwave irradiation, electron beam (e-beam) irradiation,mechanical stretching, or the like.

In another aspect, the present invention provides a method for making apermeable package. The method involves applying a physical force to apackage that includes a product and the above-described film. As before,application of the physical force to the package affects the secondmaterial which, in turn, acts on the first material so as to form holeswhich allow the product to be exposed to oxygen through the film. Filmstreated according to this method can be rendered permeable before orafter manufacture of the package through simple application of aphysical force such as those described immediately above.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The invention relates to a method for making permeable films and to amethod for making packages containing such permeable films. The methodof the present invention advantageously allows packaging films to bemade permeable at any desired point along the process of filmmanufacture, package making, shipping, and display.

According to the invention, films are made permeable by initiallyproviding a packaging film made one or more of the polymeric materialscommonly used in the manufacture of packaging films such as, forexample, any one or more of a wide variety of olefinic resins (e.g.,C₂-C₁₂ α-olefins), poly(vinyl chloride), ionomers, nylons, or other suchpolymers. Many homo- and co-polymers of ethylene are used in a widevariety of packaging films. The polymer(s) is/are referred to herein asa first phase or first material.

The film may include more than one layer. The layers of such a film canbe classified according to their purpose such as, for example,food-contact layer, sealant layer(s), abuse layer(s), bulk layer(s),oxygen barrier layer(s), moisture barrier layer(s), tie layer(s), etc.Those of ordinary skill in the art are aware of the plethora of polymersand polymer blends that can be included in each of the foregoing. Thefollowing are some non-limiting examples of combinations in whichletters are used to represent film layers:

A, A/B, A/B/A, A/B/C, A/B/D, A/B/E, A/B/C/D, A/B/C/E, A/B/E/E′, A/B/D/E,A/B/D/C, A/B/C/B/A, A/B/C/D/A, A/B/E/B/A, A/B/C/D/E, A/B/C/E/D,A/B/D/C/D, A/B/D/C/E, A/B/D/E/C, A/B/D/E/E′, A/B/E/C/E, A/B/E/C/D,A/B/E/C/D, A/B/E/D/D′, A/B/E/D/E

wherein:

A represents a food-contact layer and/or a sealant layer;

B represents a bulk layer or a sealant layer (depending on whether it ispresent as an inner or outer layer of the film);

C represents a layer including a polymer having a low permeance tooxygen and/or moisture;

D and D′ represent bulk and/or abuse layers (depending on whether theyare present as an inner or outer layer of the film); and

E and E′ represent abuse layers.

Of course, one or more tie layers can be used in any of the abovestructures. Additionally, adjacent layers may have differentcompositions.

Regardless of the structure of the film, one or more conventionalpackaging film additives can be included therein. Preferably, suchadditives do not interfere with or hinder the perforating action of theabove-described second material. Examples of additives that can beincorporated include, but are not limited to, antiblocking agents,antifogging agents, slip agents, colorants, flavorants, antimicrobialagents, meat preservatives, and the like. Where the multilayer film tobe processed at high speeds, inclusion of one or more antiblockingagents in and/or on one or both outer layers of the film structure canbe used. Examples of useful antiblocking agents for certain applicationsinclude corn starch and ceramic microspheres.

The film of the present invention preferably exhibits a sufficientYoung's modulus so as to withstand normal handling and use conditionsof, for example, at least about 200 MPa, more preferably at least about300 MPa, and most preferably at least about 400 MPa. (Young's modulus ismeasured in accordance with ASTM D 882, the teaching of which isincorporated herein by reference.)

The film may exhibit a shrink tension in at least one direction of atleast about 0.33 MPa, more preferably at least about 0.67 MPa. The filmpreferably exhibits a shrink tension of from about 0.67 to about 3.5MPa, more preferably from about 1 to about 3 MPa, and most preferablyfrom about 1.75 to about 2.75 MPa.

The film may be sequentially or biaxially oriented, more preferablybiaxially oriented. Orienting involves initially cooling an extrudedfilm to a solid state (by, for example, cascading water or chilled airquenching) followed by reheating the film to within its orientationtemperature range and stretching it. The stretching step can beaccomplished in many ways such as by, for example, blown bubble ortenter framing techniques, both of which are well known to those skilledin the art. After being heated and stretched, the film can be quenchedrapidly while being maintained in its stretched configuration so as toset the oriented molecular configuration. An oriented film can beannealed to reduce or completely eliminate free shrink in one or moredirections.

The film can be heat shrinkable. More preferably, the film is biaxiallyoriented and heat shrinkable. Even more preferably, the film isbiaxially oriented and has a free shrink at 85° C. in each of thelongitudinal (L) and transverse (T) directions of at least about 10%,preferably of at least about 15%. If heat-shrinkable, the filmpreferably has a free shrink at 85° C. in at least one direction (i.e.,the L or T direction) of from about 5 to about 70%, more preferably fromabout 10 to about 50%, and most preferably from about 15 to about 35%.At 85° C., the film preferably has a total free shrink (i.e., L+T) offrom about 5 to about 150%, more preferably from about 10 to about 125%,even more preferably from about 20 about 100%, yet further morepreferably from about 40 to about 90%, and most preferably from about 45to about 85%. (As used herein, “free shrink” refers to the percentdimensional change in a 10 cm×10 cm specimen of film when shrunk at 85°C. in accordance with ASTM D 2732, as set forth in the 1990 Annual Bookof ASTM Standards, vol. 08.02, pp. 368-71, the teaching of which isincorporated herein by reference.

The measurement of optical properties of plastic films, including themeasurement of total transmission, haze, clarity, and gloss, isdiscussed in detail in Pike, LeRoy, “Optical Properties of PackagingMaterials”, Journal of Plastic Film & Sheeting, vol. 9, no. 3, pp.173-80 (July 1993), which is incorporated herein by reference.Specifically, haze is a measurement of the transmitted light scatteredmore than 2.5° from the axis of the incident light.

The haze of a particular film is determined by analyzing it inaccordance with 1990 Annual Book of ASTM Standards, section 8, vol.08.01, ASTM D 1003, “Standard Test Method for Haze and LuminousTransmittance of Transparent Plastics”, pp. 358-63, which isincorporated herein by reference. Haze results can be obtained usinginstrumentation such as, for example, an XL211 HAZEGARD™ system,(Gardner/Neotec Instrument Division; Silver Spring Md.), which requiresa minimum sample size of about 6.5 cm². The film preferably has a hazeof less than about 20%, more preferably of less than about 15%, evenmore preferably less than about 10%, still more preferably less thanabout 7.5%, and most preferably less than about 5%.

As used herein, “thickness uniformity” refers to a percent valueobtained from the formula

 U _(t)=100−[(t _(max) −t _(min))/t _(max)]×100]

where U_(t) is thickness uniformity (calculated as a percentage),t_(max) is the measured maximum thickness, and t_(min) is the measuredminimum thickness. The maximum and minimum thicknesses are determined bytaking a number of thickness measurements (e.g., 10) at regular distanceintervals along the entirety of the transverse direction of a filmsample, recording the highest and lowest thickness values as the maximumand minimum thickness values, respectively, and computing the thicknessuniformity (a percent value) using the formula above. A thicknessuniformity of 100% represents a film with perfect uniformity, i.e., nomeasurable differences in thickness. A film in which the film t_(min) ismeasured at 45% of the film t_(max) has a thickness uniformity of only45%. The film preferably has a thickness uniformity of at least 30%,more preferably at least 50%, still more preferably at least 70%, andmost preferably at least 85%.

The film can have any total thickness as long as the film provides thedesired properties for the particular packaging operation to be used.Nevertheless, the film preferably has a total thickness of from about0.0075 to about 0.25 mm, more preferably from about 0.0125 to about0.125 mm, more preferably from about 0.025 to about 0.1 mm, even morepreferably from about 0.0375 to about 0.09 mm, and most preferably fromabout 0.045 to about 0.075 mm.

The film can be irradiated (by, e.g., subjecting it to radiation from ahigh energy source such as a high energy electron generator) so as toalter the surface of the film and/or induce crosslinking betweenmolecules of the polymers contained therein. The use of ionizingradiation for crosslinking polymers present in a film structure isdisclosed in, for example, U.S. Pat. No. 4,064,296 (Bornstein et al.),the teaching of which is incorporated herein by reference.

If desired or necessary to increase adhesion to an enclosed meatproduct, all or a portion of the film can be corona and/or plasmatreated. These and similar oxidizing treatments involve bringing a filmmaterial into the proximity of an O₂- or N₂-containing gas (e.g.,ambient air) which has been ionized. Various forms of oxidizingtreatments known to those of ordinary skill in the art can be used totreat an outer surface of a thermoplastic film material. Exemplarytechniques are described in, for example, U.S. Pat. No. 4,120,716(Bonet) and U.S. Pat. No. 4,879,430 (Hoffman), the disclosures of whichare incorporated herein by reference. Where the film is intended toadhere to an enclosed proteinaceous food product, regardless of whetheror not the film is subjected to such an oxidizing treatment, at leastthe inside (i.e., protein contact) layer thereof preferably has asurface energy of at least about 0.032 J/m², more preferably at leastabout 0.034 J/m², even more preferably at least about 0.036 J/m², stillmore preferably at least about 0.038 J/m², yet still more preferably atleast about 0.040 J/m², even further more preferably at least about0.042 J/m², and most preferably at least about 0.044 J/m².

In another embodiment, especially where the film is to be used withwhole muscle products, the food-contact layer of the film preferably isrelatively non-polar. In such applications, providing a food-contactlayer with a low surface energy can be desirable so as to avoid pullingoff chunks of the whole muscle product when the film is stripped fromthe product. In such instances, the surface energy of the layer inquestion preferably is less than about 0.034 J/m², more preferably lessthan about 0.032 J/m², and most preferably less than about 0.030 J/m².

Where the film is to be used in a cook-in application, it preferably cansurvive cooking for at least two hours without undergoing delaminationor seal failure at about at least 65° C., more preferably at about atleast 70° C., even more preferably at about at least 75° C., still morepreferably at about at least 80° C., and most preferably at about atleast 85° C. Preferably, the film of the present invention is capable ofsurviving cooking at the foregoing temperatures for at least about 3hours, more preferably at least about 5 hours, and most preferably atleast about 8 hours.

During film manufacture, particles (i.e., the second material) areincorporated into the film. The particles are selected of a materialdifferent from that of the film. The material(s) from which theparticles are made herein is referred to as a second phase or secondmaterial. The second material is selected so that it has a highersusceptibility to a physical force than the first material of the film.Due to this specific selection of properties, the application of thespecific physical force to the film serves to preferentially affect theparticles while having little or no direct affect on the polymericmaterial(s) from which the film is formed. Depending upon the nature ofthe particles and physical force, the particles may be heated, rapidlymoved, removed or the like so as to act locally upon the film andincrease permeability thereof through the provision of relatively smalland substantially uniform perforations through the film. The particlesin turn act locally upon the film so as to create holes or perforationsin the film and thereby increase permeability thereof. If desired, theperforations can be made relatively small and substantially uniformthrough the film.

The particles can be provided of a solid material, for example metal,carbon black, ferromagnetic material, or mixtures thereof. Particlespreferably have a size which can produce the desired size ofperforations to be formed in the film and a diameter or sizesubstantially the same as the film thickness. For example, particles canbe provided having a size of less than or equal to about 13 μm (0.5 mil)in a film having a thickness of about 13 μm (0.5 mil); resultingperforations can have a diameter of approximately 75 μm (3 mil). Theresulting perforations can be substantially uniform, typically at leastabout 80% of the perforations being within approximately 0.04 mm of themedian perforation size.

Particles advantageously can be loaded into the film layer(s) to berendered permeable in an amount between about 0.4% and about 4%,preferably between about 0.5% and about 3.5%, more preferably betweenabout 0.6% and about 3%, with all of the foregoing percentages beingbased on weight of the film. Particles can have a diameter of betweenabout 2.5 μm (0.1 mil) and about 25 μm (1 mil), preferably between about5 μm (0.2 mil) and about 18 μm (0.7 mil). Film thickness and particlesize can be selected to be substantially the same, and the film layerpreferably has a thickness of less than or equal to about 50 μm (2mils).

The particles can be incorporated into any layer(s) of a film as long asthey are able to form the desired size and number of perforations in thedesired layer(s) of the film. Of course, where the particles usedinclude one or more which are not governmentally approved for foodcontact, such particles preferably are not present in the inside layerof a film used to package food items.

As set forth above, incorporation of particles in the film serves toprovide a film which can be treated with a physical force to which theparticles are susceptible. Application of this force affects and/orremoves the particles so as to perforate the film and increase thepermeability thereof. Examples of physical forces which can be used inthe method of the present invention include spark discharge, inductiveheating, infrared heating, magnetic force, ultrasonic excitation,microwave irradiation, e-beam irradiation, laser irradiation, mechanicalstretching, or combinations thereof. Particular combinations ofparticles and useful physical forces include metal particles acted uponby inductive heating so as to increase the surface temperature of themetal particles, thereby melting a hole in the film. Also advantageouslyused together are carbon black particles which can be subjected toinfrared heating whereby the particles are heated and melt the film.Ferromagnetic particles can be incorporated into the film and acted uponusing a magnetic force, for example a magnetic induction force, so as tocompletely remove the particles from the film and thereby provideenhanced permeability as compared to non-treated film or film treatedwithout particles.

Some films may include additives such as slip and/or antiblock agents(which typically are applied to or migrate to outside surfaces of thefilm) which make conventional perforation difficult or unpredictable.However such films are readily perforated using high voltage spark orspark discharge on films including particles as set forth above, whichitself is an improvement. Perforations thus formed have a small size,for example less than or equal to about 0.1 mm (4 mil), preferably nomore than about 75 μm (3 mil). Such perforations can be substantiallyuniform. Other combinations of particles and forces can be envisioned bythe ordinary skilled artisan.

Specific parameters such as magnitude and duration of force to beapplied depend upon the polymeric material(s) used, the size and spacingof particles, film thickness and the like. These variables can bereadily determined by a person of ordinary skill in the art so as toprovide the desired preferential application of force to particles whichin turn act locally on the material of the film so as to form thedesired holes in the film without otherwise damaging same.

In each of the aforesaid examples, application of the physical force canbe carried out during film manufacture, during package manufacture, orafter package manufacture, for example shortly prior to placing a meatpackage on the shelf. Regardless of when perforations are formed, theenhanced permeability can allow oxygen to reach a packaged meat productas frequently is desired.

In accordance with the present invention, application of the physicalforce can be carried out while the film is under tension, eithermonoaxially or biaxially. This advantageously serves to provide for theformation of perforations which, after tension on film is released, havea smaller size.

The method of the present invention advantageously can be carried out inthe manufacture of single or multilayer films or structures. Forexample, one layer of a multilayer film advantageously may be providedhaving particles incorporated therein in accordance with the presentinvention. This film could be a barrier layer or film of a multilayerstructure having oxygen barrier properties. Upon application of thephysical force in accordance with the present invention, the barrierlayer is perforated so as to render the multilayer structure morepermeable. As described previously, this can be used in an environmentfor the packaging of fresh red meat products.

Alternatively, the packaging film of the present invention can bemanufactured and incorporated into a peelable film structure if desired.In multilayer films, those which include a layer containing particlescan be manufactured using co-extrusion, lamination and various otherfilm manufacturing techniques known in the art.

As set forth above, the method of the present invention advantageouslymay be used to provide films and packages including such films whereinpermeability can be created in the film during film manufacture, duringpackaging, or at some stage thereafter. Films incorporating particles asset forth above often have very good optical properties includingclarity, and the particles can be nearly invisible to the naked eye.

The present invention is not limited to the illustrations described andshown herein, which are deemed to be merely illustrative and which aresusceptible to modification of form, size, arrangement of parts anddetails of operation. The invention rather is intended to encompass allsuch modifications which are within its spirit and scope as defined bythe claims.

I claim:
 1. A method of increasing the permeability of a filmcomprising: providing a film having a thickness of from about 0.3 mil toabout 9.8 mils; at least one layer comprising a first materialcomprising a polymeric material selected from the group consisting ofolefinic resins, poly(vinyl chloride), ionomers, nylons, and homo- andco-polymers of ethylene; and a plurality of solid particles incorporatedinto the at least one layer, wherein the solid particles: comprise asecond material different from the first material; and have a highersusceptibility than the first material to a selected physical forceselected from the group consisting of spark discharge, inductiveheating, infrared heating, magnetic force, ultrasonic excitation,microwave irradiation, e-beam irradiation, and laser irradiation; andapplying the selected physical force to affect the particles and createperforations in the at least one layer, whereby the permeability of theat least one layer is increased.
 2. The method of claim 1 wherein theselected physical force comprises inductive heating.
 3. The method ofclaim 1 wherein the selected physical force comprises magnetic force. 4.The method of claim 1 wherein the selected physical force comprisesultrasonic excitation.
 5. The method of claim 1 wherein the selectedphysical force comprises microwave irradiation.
 6. The method of claim 1wherein the selected physical force comprises e-beam irradiation.
 7. Themethod of claim 1 wherein the selected physical force comprises laserirradiation.
 8. The method of claim 1 wherein: the film comprises amultiple layer film; and at least one of the layers is an oxygen-barrierlayer incorporating the plurality of solid particles in an amount ofbetween about 0.4% and about 4% based on the weight of the film, whereinthe particles have a diameter of between about 2.5 μm and about 25 μm.9. The method of claim 1 wherein the film comprises at least two layers.10. The method of claim 1 wherein: the film comprises at least twolayers; and the plurality of solid particles are incorporated into onlyone layer of the at least two layers.
 11. The method of claim 1 wherein:the film comprises at least two layers; and at least one of the layersis an oxygen-barrier layer incorporating the plurality of solidparticles.
 12. The method of claim 1 wherein the film comprises apeelable film.
 13. The method of claim 1 wherein the film has a Young'smodulus as measured by ASTM D882 of at least about 200 MPa.
 14. Themethod of claim 1 wherein the film comprises an oriented film.
 15. Themethod of claim 1 wherein the film comprises a heat-shrinkable filmhaving a total free shrink of from about 5% to about 150% at 85° C. inaccordance with ASTM D2732.
 16. The method of claim 1 wherein the atleast one layer incorporates the particles in an amount of between about0.4% and about 4% based on the weight of the film.
 17. The method ofclaim 1 wherein the at least one layer incorporates the particles in anamount of between about 0.6% and about 3% based on the weight of thefilm.
 18. The method of claim 1 wherein the second material comprisesmetal.
 19. The method of claim 1 wherein the second material comprisesmetal and the selected physical force comprises inductive heating. 20.The method of claim 1 wherein the second material comprises iron and theselected physical force comprises inductive heating.
 21. The method ofclaim 1 wherein the second material comprises carbon black.
 22. Themethod of claim 1 wherein the second material comprises carbon black andthe selected physical force comprises infrared heating.
 23. The methodof claim 1 wherein the second material comprises ferromagnetic materialand the selected physical force comprises magnetic force.
 24. The methodof claim 1 wherein the selected physical force is applied while the filmis under tension.
 25. The method of claim 1 wherein the selectedphysical force is applied to create the perforations while the film isunder tension and further comprising subsequently releasing the filmfrom the tension to reduce the size of the perforations.
 26. The methodof claim 1 wherein the particles have a diameter of between about 2.5 μmand about 25 μm.
 27. The method of claim 1 wherein the particles have adiameter of between about 5 μm and about 18 μm.
 28. The method of claim1 wherein the particles have a diameter of less than about 5 μm.
 29. Themethod of claim 1 wherein the particles have a diameter of less than orequal to the film thickness.
 30. The method of claim 1 wherein theparticles have a diameter substantially the same as the film thickness.31. The method of claim 1 wherein the at least one layer has a thicknessof less than or equal to about 2 mils.
 32. The method of claim 1 furthercomprising forming a package comprising the film before applying thephysical force to the film.
 33. The method of claim 1 further comprisingforming a package comprising the film and enclosing a product, beforeapplying the physical force to the film.
 34. The method of claim 1further comprising forming a package comprising the film and enclosing ameat product, before applying the physical force to the film.
 35. Themethod of claim 1 wherein the selected physical force removes theparticles from the at least one layer to create the perforations.